There have been several incidents in which a helicopter approaching a raised landing zone (LZ) has descended below the LZ altitude without receiving any warning of too great a sink rate. Because a helideck is raised (often a few hundred feet above the ground or ocean), the existing sink rate alert function doesn't provide a timely alert. In the current ground proximity warning system (GPWS) for helicopters, the sink rate alert envelope is determined using only radio altitude information. The aircraft's radio altitude continues to read a large value (300 feet, for example) until the helicopter gets over the helideck. At that point, the radio altitude jumps to 20 feet, for example. Often, the helicopter is sinking toward the helideck at that point, and there may not be sufficient time for the pilot to react if the sink rate is greater than optimal.